Saut-du-Saumon orthogneiss
The Saut-du-Saumon orthogneiss is an orthogneiss that emerged from a former granite in the Thiviers-Payzac unit of the north-western Massif Central ( France ).
designation
The Saut-du-Saumon-Orthogneiss was named after its type locality , the Saut-du-Saumon - a terrain level of the Gorges du Saillant gorge in the Vézère valley .
geography
The pre-kinematic Saut-du Saumon orthogneiss is an octopus-like, 15 kilometers long and only 0.5 to 1 kilometer wide, northwest-southeast-trending rock body, whose elongated tail begins at Donzenac in the south, and whose 5 to 7 tentacles in the vicinity end of Orgnac-sur-Vézère near the Loyre . Inside it has elongated septa and ribbons of Thiviers sandstone in the decameter range, which run parallel to the foliation . Its southeast end disappears below the Briver basin (basin of Les Saulières ), where it is covered by upper Carboniferous sediments, but can be geophysically tracked for another 1.5 kilometers in strike direction.
Petrology
The Saut-du-Saumon orthogneiss has porphyroclastic alkali feldspar eyes in the sub-centimeter range (so it is an eye gneiss ). Only at the northwestern end of the tentacles near Orgnac can still relatively undeformed apophyses with the original granite, here only cataclasically stressed, be recognized. Continuous facies transitions confirm that the orthogneiss emerged from granite. Occasionally, aplite and micro-granite are also associated with orthogneiss, for example in Orgnac, Les Trois Villages and Donzenac .
mineralogy
The gray-blue, uniform-grain, slightly porphyry orthogneiss shows the following mineralogical structure:
- Microcline and subordinate plagioclase function as porphyroclastic, sometimes quite coarse-grained eyes
- the fine-grained matrix consists of quartz , feldspars and mica .
The microcline is subautomorphic, cube-shaped and mostly perthitic. It forms monocrystalline porphyroclasts , which are often sheared on parallel fractures on the left side. Larger single crystals are often crowned by smaller grains and internal fractures are healed by newly formed quartz. Plagioclase (An 21-30 ), which occurs primarily as a matrix mineral, is also subautomorphic and often zoned. As a porphyroclast, it is found in the tonsils, which can be arranged either parallel or at an angle to the foliation . Usually more broken up than Mikroklin, it is also coated with subgrains. It is also covered with subautomorphic muscovite and epidote . The quartz underlines the foliation and appears as flattened bands. In the latter, even-grain quartz crystals form a puzzle. Often millimeter-sized quartz relics can also be found as gusset fillers. The subautomorphic, red-brown biotite has preferably aligned parallel to sigmoid shear ligaments. Streamlined biotite shows adhesions with muscovite. Relic biotite can be strewn with rutile . Accessories are muscovite, zircon , apatite and opaque minerals. Chlorite appears in sbc shear bands that shift to the right and is therefore a late formation (conversion of biotite).
Chemical composition
| Oxide wt.% |
Saut-du-Saumon-Orthogneiss Orgnac |
Saut-du-Saumon orthogneiss | Estivaux granite | Corgnac granite |
|---|---|---|---|---|
| SiO 2 | 71.60 | 70.00 | 71.00 | 68.00 |
| TiO 2 | 0.44 | 0.47 | 0.25 | 0.54 |
| Al 2 O 3 | 13.50 | 14.10 | 15.30 | 14.45 |
| Fe 2 O 3 | 0.27 | 0.10 | 0.55 | 4.21 dead |
| FeO | 3.10 | 3.70 | 1.20 | |
| MnO | 0.05 | 0.06 | 0.03 | 0.06 |
| MgO | 0.90 | 1.10 | 0.85 | 1.17 |
| CaO | 1.65 | 1.70 | 1.40 | 1.51 |
| Na 2 O | 3.50 | 3.80 | 4.70 | 3.77 |
| K 2 O | 3.65 | 4.10 | 4.20 | 3.01 |
| P 2 O 5 | 0.20 | 0.21 | 0.09 | |
| H 2 O - | 0.30 | 0.05 | 0.10 | |
| H 2 O + | 0.40 | 0.80 | 0.65 |
The Saut-du-Saumon-Orthogneiss has the chemical composition of a calcareous, Monzonitic granite. It differs only slightly from the other granites of the Thiviers-Payzac unit - it has a higher CaO and P 2 O 5 content - and a lower Al 2 O 3 content.
deformation
.JPG)
The orthogneiss originally comes from a porphyry granitoid that was plastically deformed after crystallization in the Middle Ordovician . Undeformed areas cannot be seen. The entire rock body is intensely penetrated by a vertical, NW-SE-trending foliation. It also shows horizontally aligned, also NW -SE-trending linear lines, the rock can therefore be referred to as SL tectonite . At its edges to the neighboring rocks (Thiviers sandstone and Donzenac slate ) it forms mylonite and ultramylonite with left-sided shear sense. Inside it, both dextral and sinistral shear senses overlap in a non-coaxial manner.
The tectonic stress on the rock can be seen in undulant extinguishing quartz, in the formation of undersized grains and in frayed, jagged grain boundaries that reveal significant recrystallization . As shear criteria, asymmetrical quartz pressure shadows, pressure shadows around microcline and generally shear bands are used. However, due to the superposition of the two shear systems within the orthogneiss, the sense of shear cannot always be clearly defined. This is also clear from the quartz c-axes, which show a symmetrical distribution.
Age
The original intrusive age of the Saut-du-Saumon orthogneiss is given as the Middle Ordovician (470 to 460 million years - Acadian phase ). According to Bernard-Griffiths and colleagues (1977), the Mylonitic movements occurred at the turn of the Devonian / Carboniferous 361 million years ago.
See also
swell
- Jacques Grolier and others: Feuille Tulle . In: Carte géologique de la France à 1/50000 . BRGM.
- Roig, JY: Evolution tectono-métamorphique d'un segment de la chaîne hercynienne. Rôle du plutonisme dans la caractérisation des tectoniques du Sud-Limousin (Massif Central français) - Doctoral thesis . Univ. Orléans, 1997, p. 287 .
Individual evidence
- ↑ Bernard-Griffith, J., Cantagrel, JM and Duthou, JL: Radiometric evidence for an Acadian tectonometamorphic event in western Massif Central français . In: Contrib. Miner. Pet. Band 61 , 1977, pp. 199-212 .